The invention relates generally to measuring vehicle speed and more specifically to a method and system for measuring vehicle speed and filtering the measurement for display on a vehicle speed display.
In current vehicle speed measurement systems, vehicle speed is derived using a pulse period measurement method. Such a system typically consists of a gear having a plurality of teeth, a magnetic sensor, and signal conditioning circuitry. The gear rotates at a speed that is directly related to the speed of the vehicle. As the gear rotates, the gap between the sensor and the gear increases and decreases as each tooth passes by. This movement causes an electric current to be induced in the magnetic sensor and is converted by the signal conditioning circuitry into a periodic speed signal. Vehicle speed is then derived from this signal by measuring the period of the signal and employing an equation which relates the period of the signal to vehicle speed.
Ideally, vehicle speed using this method can be calculated from a single period measurement. In practice, however, such a single period measurement will not produce an accurate speed measurement because of variances in tooth periods caused by manufacturing tolerances relating to gear concentricity as well as tooth height and width.
To address the accuracy problems associated with tooth to tooth period variations, current systems attempt to average out the effects of the gear manufacturing process by calculating an average pulse period over a full revolution of the gear. A problem associated with the averaging technique, however, is that at low vehicle speeds, such as below 10 mph, the pulse period measurements take much longer to update. Accordingly, the last speed measurement remains displayed for longer and longer periods until an updated measurement is generated. This can result in undesirable display jitter in the low speed range. To avoid this problem, some systems have simply set the display stop set somewhere around 10 mph.
Another problem associated with averaging is that at higher speeds, the effects of manufacturing tolerances become more pronounced and also may result in display jitter.